1. Field
This invention relates to ion exchange separation. It is specifically directed to such a process conducted in a simulated moving bed of sorbents.
2. State of the Art
Ion exchange systems which involve the passing of liquids containing at least two components through an ion exchanger are well known. Such processes are sometimes referred to as adsorber systems, and the process whereby one dissolved constituent is separated from another dissolved constituent by passage through an ion exchanger or adsorber bed is sometimes referred to as "adsorbtive separation". These processes generally involve passing a solution through a bed of resin, whereby one constituent is attracted to the resin bed, followed by an elution, or regeneration, step. The elution step removes the adsorbed constituent from the resin as "extract." The solution from which the adsorbed constituent has been removed is referred to as "raffinate," or sometimes "ashes". A common procedure is to contact the adsorber bed (ion exchange resin) alternately with the feed stock (solution of constituents) and eluant, respectively, to achieve this separation. The feed stock and eluant may flow in either co-current or counter-current relationship through a stationary bed. Use of a stationary bed has limited ion exchange separations to batch operation.
Efforts have been made to conduct ion exchange processes in a fashion which simulates the characteristics of a continuous operation. One such approach has been physically to move the exchange resin, either continuously or by periodic pulsing, from one zone to another zone. Each zone is then operated continuously, either in an adsorber (or loading) cycle, or an elution (or de-sorbing) cycle. Mechanical wear, such as that caused by friction on the individual particles of resin, has been destructive. Accordingly, processes involving the physical movement of the ion exchange resin have not gained wide acceptance.
U.S. Pat. No. 2,985,589 (Broughton et al) discloses a continuous sorption process employing a stationary or fixed bed in a fashion which simulates a moving bed. Stationary bed operated by such procedures are commonly referred to as "psuedo-moving beds," or more often, as "simulated moving bed". A more recent patent disclosing a simulated moving bed process in U.S. Pat. No. 4,182,633 (Ishikawa et al).
The simulated moving bed of the prior art generally is constructed as a single column partitioned into a plurality of individual compartments. These individual compartments may be regarded as zones, connected in series with an inlet at the top of each zone and an outlet at its bottom. The process is regarded as continuous because a continuous circulation flow is maintained through the zones in series, being collected at the bottom of the last zone in the series, after having percolated through each of the zones beginning with the zone at the top of the column. The collected liquid is re-introduced at the top of the column to the first zone of the series for recirculation. The inlets and outlets of each zone in the system are connected by means of an exterior manifold with appropriate valving to selectively introduce feed stock or eluant to the top of any zone, and to withdraw raffinate or extract from the bottom of any appropriate zone. Each zone may thus function in turn as the sorption zone, the displacement zone, the elution zone and the rinse (or regeneration) zone. The function of the zone is established by the nature of the medium which is either entering or leaving the zone at any particular moment.
It is an important objective in the operation of a simulated bed to maintain well defined interfaces between the various phases flowing through the system. Although the zones are ordinarily treated in batch fashion, the interface between adjacent phases progresses through the system continuously. For this reason, it is important to know or be able to predict when the front of the raffinate phase or extract phase has moved to the proximity of an outlet associated with one of the zones. The arrival of the front at an outlet should correlate with the opening of the manifold valve associated with that outlet and the closing of the outlet valves of the other zones. Opening of the appropriate outlet manifold valves must be coordinated with the opening of inlet valves for the introduction of feed stock and eluant to the tops of the appropriate zones within the system. Introduction of these liquids is desirably done in a fashion which maintains an undisturbed interface between the liquid phases flowing through the column. Various expedients are known for this purpose. For example, liquids may be introduced through various distributor systems which inject liquid across substantially the entire cross-section of the vessel. It is also known to withdraw fluids from an outlet through a similar distribution system.
The simulated moving bed processes which have evolved suffer from a number of inherent shortcomings. For example, the use of a bed of ion exchange resin always requires periodic backwashing of the sorbent material, both for the removal of fines and to loosen the bed. After a period of operation, the inevitable compaction of the resin causes an intolerable pressure drop across the bed. A compacted bed impedes the percolation of liquid through the column. Moreover, it is important that the beds confined within each zone of a column be classified periodically into layers of equal particle size. Otherwise, it is impractical to maintain approximately equivalent conditions within each zone. It is also important periodically to remove entrained gas pockets within the bed, because they tend to disturb the desired even cross-sectional fluid flow through the bed. It is not practical to backwash the beds confined within individual zones individually. Accordingly, it is difficult to maintain optimum flow conditions through the bed. When backwashing becomes critically important, the entire bed must be removed from the column and replaced.
U.S. Pat. No. 4,001,113 discloses an ion exchange treating system in which two or more exchangers or adsorber vessels are connected in series, and each vessel is filled with ion exchange resin leaving sufficient free board to allow for expansion of the medium. The process disclosed does not involve a simulated moving bed. Each vessel is provided with an expansion chamber positioned directly above it to accept resin during backwashing procedures. Each vessel is also provided with distribution systems at the top and bottom for the introduction or withdrawal of liquids.
For optimum efficiency and excellent chromatographic separation within an ion exchange column, the sorbent bed should be as high as is practical. That is, the flow path through the zone is desirably long. Placement of a plurality of zones within a column imposes a cumulative pressure drop on the system which inevitably restricts the practical height permitted for each of the beds within the several zones of the column.
Successful operation of a simulated moving bed process depends upon the maintenance of steady state equilibrium, as reflected by the absence of drift in the concentration gradient of the various components to be separated and in the fractions collected from the circulating loop. As previously indicated, it is important to maintain well defined fronts for the various phases flowing through the column. It is also important reliably to predict the progress of these fronts through the column. This prediction is correlated both to the establishment of a circulation flow rate within the loop and to the timing of shifting the opening and closing of the inlet and outlet ports connected to the manifold system. Heretofore, establishing both the circulation flow rate and the timing of the manifold flow control have been based upon either trial and error or involved measurements of component concentrations. The aforementioned U.S. Pat. No. 4,182,633 discloses one approach to controlling a simulated moving bed process which involves measurements and rather complex computations.
It is characteristic of simulated moving bed processes that the volumes of the streams (feed stock and eluant) entering the circulating loop do not precisely equal the volumes of the streams (extract and raffinate) leaving the circulating loop. Moreover, the total pressure drop through the column is the sum of a pressure drop through the various zones, each of which can vary significantly. This total pressure drop must be balanced against the pressure generated by a recirculation pump required by the system to transfer the circulating stream collected at the bottom of the column back to the top of the column. All of these factors make it extremely difficult to maintain pressure integrity within the loop and to differentiate the causes of any pressure imbalance within the loop.
Simulated moving bed processes may utilize adsorbents or exchangers selected from any of the known compounds or classes of compounds known to the art. The resin beds may be either organic or inorganic in nature, having functionality either as an ion exchanger or adsorber. The dissolved constituents which may be separated by simulated moving bed techniques also include either organic or inorganic constituents dissolved in a medium which is most commonly liquid, but could conceivably be gaseous. The ideal application of a simulated moving bed process is to separate constituents which are similar in chemical and physical nature. Accordingly, they are of special usefulness in separating compounds such as glucose and fructose from an aqueous invert sugar solution or fructose from a solution which also contains starch hydrolysates.